Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD) A constituent element of an ELD includes such as an inorganic electroluminescence element (hereinafter, referred to as an inorganic EL element) and an organic electroluminescence element (hereinafter, referred to as an organic EL element). An inorganic EL element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescence element is an element provided with a constitution comprising an emitting layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a hole being injected into the emitting layer to be recombined, resulting emission utilizing light release (fluorescence and phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescence element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
Further, the major feature of the organic electroluminescence elements is also in the form of a surface light source of thin film differing from conventionally employed main light sources such as a light emitting diode or a cold-cathode tube. Possible applications, which can effectively utilize the above characteristic, include light sources for lighting and backlights of various displays and a light source for illumination. In particular, it has been attracted attention to employ them as a light source for illumination in recent years.
In the past, a glass has been used from the viewpoints of high thermal stability, high transparency, and low water vapor permeability as a substrate for such an element. However, the glass has features of easily broken and relatively heavy by nature, and in order to use for various applications, it has been required a light weight substrate which is h flexible and hardly broken. Consequently, use of a transparent plastic substrate has come to attract attention. However, during production, transport and keeping of an element, a plastic substrate is easily given a graze or a scratch on its surface by various external forces. As a result, it may not only spoil the external appearance, but it may produce problems such as nonuniform luminescence in the light emitting surface of the light emitting element and change of luminescence properties, and production of a crack originated from these scratches at the time of bending.
Moreover, when a light emitting element is composed, it is common practice to arrange a transparent electrode on a plastic plate to. Though, it may occur a crack on an electrode portion starting from the part bent from the flexible nature, or it may produce peeling of an electrode layer arises for the thermal shrinkage which is the characteristics of a plastic, and there is a problem of causing a leak and a short circuit of an element.
On the other hand, as a problem which should be solved for improving the performance of an organic electroluminescence element, there has been a problem for a long time that taking out efficiency of the light (the ratio of the energy of the taking out light from the substrate to the energy of the emitting light) was low. That is, since there is no directivity in luminescence of a light emission layer and the luminescence dissipates in all directions, there is a problem that the loss at the time of leading a light to front direction from the light emission layer is large, light intensity is insufficient, and a display screen becomes dark.
With regard to the luminescence from a light emission layer, only the luminescence coming out forward is used. The taking out efficiency of the light from the front can be approximated as “½n2” according to the multiple reflections based on the classic optical theory. It will be mostly determined be the refractive index “n” of a light emitting layer. If the refractive index “n” of a light emitting layer is set to be about 1.7, the luminescence efficiency from the above-mentioned organic EL part will become simply about 20%. The remaining lights will spread to the surface direction of the light emitting layer (disappearance to the lateral direction) or will disappear on the metal electrode facing transparent electrode sandwiching the light emitting layer (absorption to the back direction)
To put it in other wards, the usual organic electroluminescence element will emit light inside the layer whose refractive index is higher than air (a refractive index is 1.7 to about 2.1), and can take out only about 15% to 20% of the lights generated in the light emitting layer of light. This caused the followings. The light which enters into an interface (interface between a transparent substrate and air) with the angle beyond a critical angle θ will cause total reflection and cannot take out the light outside the element. The light caused total reflection between the transparent electrode or the light emitting layer and the transparent substrate, and the light was wave-guided through the transparent electrode or the light emitting layer, and as a result, the light escaped in the surface direction of the element part, and there occurred loss of the light.
It is known the technology of providing with the hard coat layer made of a curable resin on the surface of a plastic substrate to improve a scratch proof nature of a plastic substrate surface. For example, in Patent document 1, there is disclosed a technology to provide with a hard coat layer made of a photo- or thermo-curable resin on at least one surface of a polycarbonate substrate.
As a means to prevent the appearance of crack in the electrode layer or the peeling from the substrate by improving the close adhesion of the substrate with the electrode, there is disclosed a technology, for example, in Patent document 2, to form an interlayer having an inorganic compound film and a graft polymer layer combined therewith on the uppermost surface of the substrate.
However, when these technologies are performed, it will be accompanied with increase of the cost due to the fact that the manufacturing process becomes complicated. In addition, fine adjustment of the technical constituent elements is required and it is requested an improved means which is easily carried out and by which adhesion with a substrate is more improved.
Various methods are examined as an approach of improving an efficiency of taking out of lights. For example, in Patent document 3, there is reported a method which forms ruggedness on a surface of a transparent substrate for preventing the total reflection at the interface between a transparent substrate and air from occurring. In Patent document 4, there is reported a method which introduces a flat layer between a substrate and a light emitting body, the refractive index of the flat layer being adjusted to be intermediate value of the substrate and the light emitting body, to form an anti-reflection film. Furthermore, in Patent document 5, there is reported a method of introducing a flat layer between a glass substrate and a light emitting body, the flat layer having a smaller refractive index than the glass substrate. In Patent document 6, there is reported a method of forming a diffraction grating at any one of places between a glass substrate, a transparent electrode layer, and a light emitting layer (including between the glass substrate and the external environments).
However, in the method of forming ruggedness on the surface of the transparent substrate or in the composition of forming a diffraction grating, although it is common to use the method of preparing ruggedness by etching with the photo lithography as a means to form ruggedness, this way has low manufacturing efficiency and it becomes cost increase. Moreover, by the method of introducing a flat layer having an intermediate refractive index between the substrate and the light emitting body, or by the method of introducing a flat layer having a lower refractive index than the glass substrate, there will exist, after all, an interface having a different refractive index, and there is little improvement in the taking out efficiency of lights.
Furthermore, as a combination of the above-mentioned technologies, the following organic EL display is disclosed in Patent document 7, for example. In one side of an organic EL element, a transparent material layer is formed through a minute ruggedness layer having a plurality of minute ruggedness with a pitch of 400 nm or less. This minute ruggedness layer is arranged to be located in the position opposing to the side of entering the light from the organic EL element. This minute ruggedness layer also serves as a hard coat layer.
However, the manufacturing process was complicated such as to form a rugged layer, to past this to a transparent material layer, and further to paste this so that the rugged portion will oppose to the substrate of the element. In addition, since the rugged portion is facing to the substrate of the element, an external force will be concentrated locally on the substrate through the convex part, and it may occur a problem of damaging the surface of the substrate.
It has been requested an improving means which enables to improve the followings at the same time: anti-scratch property of a surface of a plastic substrate; close adhesion between a substrate and an electrode layer; and taking out efficiency of light, without needing preparation of complicated element composition or plural component members.